The interest in synchronous reluctance motors is increasing as it might become a candidate for replacing a conventional induction motor. In order to fully exploit the capabilities of the synchronous reluctance motor, a frequency converter is needed and a closed-loop control should be performed. However, closed-loop controls (especially speed-sensorless controls) have to be provided with appropriate parameters to avoid instabilities and to work properly. Usually, the parameters required by the control are obtained through a series of experimental tests.
Nowadays, these tests are performed automatically by the frequency converter with a minimal intervention by an external human operator. Different test signals on the machine and post-processing methods are exploited in order to estimate and complete the set of the parameters. These automatically performed tests are generally referred to as “self-commissioning” or “ID-run”.
One of the major benefits of ID-runs is the possibility to conduct standstill tests, during which the machine is at complete standstill and different signals are injected. In this operating mode, maximum safety is obtained, and the motor can be tested on-site with direct connection to a mechanical load. This is particularly beneficial when the application is re-vamped and only the frequency converter is replaced, while leaving the existing motor. In this case, there is no need to remove the motor from the plant.
In the specific case of the synchronous reluctance motor, some issues arise. The machine has a strongly non-linear relation between current and flux linkages, with saturation effects and cross-magnetization effects more pronounced.
An example is shown in FIG. 1, which illustrates the current-to-flux linkage curves for a synchronous reluctance motor obtained from the finite element method analysis.
In FIG. 1, λd and λq are the flux linkages in the d and q axes, respectively, and id and iq are the corresponding currents, i.e. a direct axis current component and a quadrature axis current component, respectively, of a motor current vector. The derivatives of λd and λq with respect to id and iq, respectively, return the value of the inductances Ld and Lq. Inductances Ld and Lq on d and q axes, respectively, are dependent on both currents id and iq. In practice, Ld will to the greatest part depend on id but also to a smaller extent on iq. This is referred to as a cross-coupling cross-magnetization effect.
For a correct closed-loop (speed-sensorless) control of the machine, knowledge of the inductances in any operating point is beneficial. The inductance is generally defined as the ratio of the flux linkage over the current; depending on the adopted control strategy, apparent inductances (ratio between large-signal values) or differential inductances (ratio between small-signal values) might be needed. In any case, it is clear from FIG. 1 that the inductances vary as a function of the operating point.
The left-hand side of FIG. 1 shows the flux linkage on the d axis, while the right hand side of FIG. 1 shows the flux linkage on the q axis. Saturation is more visible on the d axis due to the presence of more iron material in the magnetic path, while the q axis has a more “linear” profile due to more air material in the magnetic path.
From FIG. 1, it can be deducted that a truly effective ID-run should be capable of estimating the inductances in any operating point. The drawback is that for each operating point where both currents id and iq are different from zero, an electromagnetic torque is produced and the motor starts rotating (if the mechanical load allows it), according to the torque and mechanical equations:
      τ    =                  3        2            ⁢              p        ⁡                  (                                                    λ                d                            ⁢                              i                q                                      -                                          λ                q                            ⁢                              i                d                                              )                          τ    =                  τ        L            +                        J          m                ⁢                              ⅆ                          ω              m                                            ⅆ            t                              +                        B          m                ⁢                  ω          m                    where p is the number of pole pairs in the machine, τ is the torque, τL is the load torque, Jm is the mechanical inertia, Bm is the viscous friction and ωm is the mechanical speed.
Current self-commissioning procedures are capable of estimating, at standstill, the inductances where either id or iq is zero, thus when no torque is produced. For all other operating points, torque ramps are induced in the motor, and the inductances are estimated during the speed transient. Such operating condition is not at standstill, and might require the motor to be disconnected from the mechanical load.